1 · Values and control flow · 25 MIN

Collections, loops and LINQ

LINQ can describe a query whose execution is deferred.

Where and Select build a query; ToArray materializes its current result. Without materialization, later enumeration may observe changes to the source. This example deliberately creates a snapshot of passing scores while leaving the source list intact. Do not modify a List while enumerating it. A clear ownership contract says whether a method returns a snapshot, a live view or a lazy sequence. That contract matters as much as choosing short syntax.

Choose the materialization boundary deliberately.

Read the example

using System;
using System.Collections.Generic;
using System.Linq;
var scores = new List<int> {59,60,90};
var passing = scores.Where(score => score >= 60).ToArray();
scores.Add(100);
Console.WriteLine(string.Join(",", passing));
Console.WriteLine(scores.Count);

60,90
4

Your challenge

Write a filter method returning a materialized snapshot and compare it with returning IEnumerable without ToArray.

Solution cost: O(n) filtering and O(k) materialized matches. time · Account for collection storage separately from the returned result. space

Common trap

A deferred query is not automatically a cached result.

using System;
using System.Collections.Generic;
using System.Linq;
var scores = new List<int> {59,60,90};
var passing = scores.Where(score => score >= 60).ToArray();
scores.Add(100);
Console.WriteLine(string.Join(",", passing));
Console.WriteLine(scores.Count);

Further reading: Official documentation